1. Field of the Invention
This invention relates to a process simulation method, a process simulator and a chemical vapor deposition system employing the same, and more particularly to a process simulation method, a process simulator and a chemical vapor deposition system employing the same wherein process conditions upon formation of a dielectric film for a capacitor by a chemical vapor deposition (hereinafter referred to simply as CVD) method can be set automatically and non-artificially.
2. Description of the Related Art
In recent years, as the degree in integration of semiconductor devices goes up, increases in number of steps and cost in a manufacturing process and increases in time and cost required for development of the process make problems. For example, in a dynamic random access memory (DRAM), in order to obtain a large capacity, a dielectric film with a uniform thickness must be formed on a capacitor having a complicated electrode structure of such type as the trench type or the hemispherical grain (HSG) type. For the formation of a dielectric film on the above mentioned electrodes, the CVD method is principally employed since it is advantageous in that it gives a good step coverage of the dielectric film which is evaluated in terms of a ratio between the film thickness on the uppermost surface of a step and that on the side wall of the step (hereinafter referred to as coverage ratio) and that it can easily achieve selective deposition upon formation of a film. In order to reduce the time and the cost required for development of such process technique, a process simulator wherein optimum conditions for achieving a desired process are automatically set to a CVD apparatus has been proposed.
Here, how to set optimum CVD condition which gives the best step coverage of a dielectric film is considered. It is known that improvement in step coverage, where the theoretical value for the best step coverage is 1, can be achieved qualitatively by reducing the amount in material gases which arrive at the film surface and contribute to formation of the film, and by reducing the surface reaction coefficient of the species which will be hereinafter described. Accordingly, when the optimum CVD condition is automatically determined by a process simulator using certain parameters and the obtained optimum condition is required to be simultaneously set to the CVD system, it is necessary that, in the process simulator, the vapor phase reactions and the film surface reactions be treated without depending on any experimental results, that is, those reactions are non-empirically described.
One of the documents which describe a representative process simulator in which a step coverage of a thin film formed by a conventional CVD process is calculated by computer simulation is disclosed in M. Ikegawa and J. Kobayashi, Journal of Electrochemical Society, Vol. 136, No. 10, 1989, pp. 2,982-2,986. According to the simulator of the document, the procedure of growth of the film is simulated using such a model as shown in FIG. 9.
In the model shown in FIG. 9, it is assumed that a process region in which a film 103 is formed on a substrate surface 102 is constituted roughly from a vapor phase and a surface/solid phase. First, in the vapor phase, material gases are decomposed by heat, light or plasma so that a molecule which is liable to react, that is, a reactant 101, is produced. The reactant 101 arrives at the substrate after colliding with other molecules, and it is absorbed on the substrate surface 102. Meanwhile, in the surface/solid phase, the reactant 101 arriving at the substrate surface 102 follows either procedure; one is that the reactant causes the reaction on the substrate surface 102 to form a film 103, and the other is that it is reflected by the substrate surface 102 without causing any reaction with the substrate surface 102 and moves to a different position on the substrate surface 102. Such procedures are calculated for a large number of individual molecules and the results of the calculations are accumulated to simulate the growth of the film 103 in such a manner as illustrated in FIG. 10.
The flow chart of the process simulation is described by referring to FIG. 10. First initial conditions for the simulation, such as kinds of material gases and a process temperature are determined or chosen (step P1). Behavior of reactants 101 in the vapor phase is calculated by a Monte Carlo method (step P2) until they arrive at the substrate surface 102 or the film 103 from positions where they have been generated. From the results of the calculation, the positions of the reactants arriving on the substrate surface and the number of the reactants 101 arriving at a position on the substrate surface are obtained (step P3). Then, it is determined whether or not the thus arriving reactants 101 react with the surface of the film 103 at the individual positions of the surface (step P4). The thickness of the film 103 is increased as the amount of the reactants 101 which have reacted with the surface of the film 103. Whether or not reaction actually occurs is determined from the value of a surface reaction coefficient set in advance. For example, where the reaction coefficient is 1.0, all of the reactants 101 arriving at the surface of the film 103 react with the surface of the film 103, but where the reaction coefficient is 0.5, half of the reactants 101 arriving at the surface react with the surface. The operations at steps P1 to P4 are repeated until the thickness of the film 103 becomes equal to a predetermined thickness tfs (step P5).
A most significant factor in the calculation described above is a reaction coefficient which indicates the probability of the reactant causing the reaction, and the reaction coefficient is determined by the energy which is need for the reactant causing the reaction. Particularly, the step coverage is influenced significantly by the reaction coefficient at the surface of the film. For example, when the reaction coefficient becomes low, since the ratio in surface reflection becomes high, reactants can reach a deep portion or the bottom of a groove, that is, a trench, of a substrate. Consequently, the coverage ratio approaches unity.
As mentioned above, the reaction coefficient is an important parameter which has a significant influence on the step coverage of the film in CVD process. According to the conventional simulation, however, the value of the reaction coefficient is set to be an empirical value, or is so determined as to fit the simulation results, in which the reaction coefficient is set as an artificial parameter, to the experimental results. Therefore, in the conventional simulation, the optimum condition for causing the coverage ratio to approach unity is a semi-experimental condition determined only from a large number of results of experiments, and this does not satisfy the requirement for a non-empirical value described above. Further, a process wherein the step coverage is improved using such a result of the simulation as described above is insufficient for supporting the development of the CVD process described above since the dominant factor of step coverage is indefinite.
In short, the conventional process simulator described above is disadvantageous in that, since an empirically estimated value or an artificial or experiential corrected value with which a result of the simulation and the result of the experiment coincide with each other is used as a value of the reaction coefficient, a process optimum condition for improving the step coverage is a semi-experiential value based on a large number of experimental results and the requirement for a non-empirical value for automatically setting a result of a simulation on the real time basis as a process condition for a CVD process apparatus is not satisfied. Further, since the dominant factor of step coverage is indefinite, the conventional process simulator is further disadvantageous in that it is insufficient as a process simulator for supporting the development of a CVD process which optimizes the step coverage of a dielectric film.